A heifer
I dont know if this is a joke or not but the correct term is heifer
Organ systems are made up of tissues which are made up of cells.
Hopefully that helps and if u need a brief explanation about that just text me.
the answer is moon. the name tells you what you can't see. since in a solar eclipse you can't see the sun that means that the moon is in between us on earth and the sun. which is why we can't see it
